Digital duplicator printing is a rotary printing process in which a stencil containing pixel image voids is mounted on the circumference of a printing drum which contains printing ink. The drum circumference is a mesh screen. Sheets of paper are passed under the drum in point contact as it rotates, ink being forced through the screens and stencil on to the paper to make a print.
It is well known to use water-in-oil emulsion inks for this printing process. The inks may contain pigment in the oil or water phase. Inks which contain pigment in the water phase can provide a number of advantages, typically better image density and reduced set-off, in comparison with similar inks containing pigment in the oil phase. Inks having water phase pigmentation are known, for example, from EP-A-704506, EP-A-827992 and EP-A-848046.
EP-A-778323 describes water-in-oil emulsion inks containing carbon black in the water phase. It is claimed that by selecting carbon black with a pH value in the range 6 to 10 it is possible to achieve excellent stability to changes such as drop enlargement and phase separation. Inks undergoing these types of changes become progressively more fluid.
EP-A-846737 describes water-in-oil emulsion inks containing carbon black in the water phase and/or the oil phase, where the pH value of the water phase is adjusted by the addition of base or buffering agent in order to achieve a value in the range 6 to 12, preferably 7 to 9. Again, the benefit claimed is stability to changes which would result in the emulsion becoming progressively more fluid.